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Old 10-04-2012, 01:07 PM   #47
Caden
Registered User
 
Join Date: Feb 2012
Location: Texas
Posts: 329
My Ride: '06 325i
Quote:
Originally Posted by jdstrickland View Post
Do not sell your car, sheesh!

I've lost track of what you have discovered so far, soory. You do not need new tires. It is poor practice to have different brands of tires on the front and rear, but this happens all of the time and everybody lives to fight another day. It is important to have two tires of the same brand on the same axle, but different brands front and rear is no big deal. If there are different brands on the same axle, especially the front, then it is likely that you might have an all-season on one side and a full on summer tire on the other, or a Z-rated on one side and an H-rated on the other. This all means that the carcass of the tires will be made differently, and this makes the rolling resistance different, and when there is different rolling resistance on the axle that does the steering, then there is a pull to one side or the other. This completely ignores the problems that come from different tread patterns that affect traction and grip, and those kinds of things.

You really need tires of the same brand and size on the same axle, but different brands or sizes on different axles is okay within defined limits.

Your car takes a tire that is essentially 25 inches in diameter, whether the wheel is a 15", 16", 17" or 18". The wheel diameter does not matter. If the WIDTH of the tire is the same front and rear, then you can change fitment from a 225/55x15 to a 225/40x18 without any problems at all. You could use the 225/55x15 on the front and a 225/40x18 on the rear without any problems -- it would look like crap, but mechanically it can be done. The 225/55x15 and 225/40x18 are within a whisker of being the same overall diameter of 25 inches, get it.

With a staggered set up -- wider tires in the rear than on the front -- then you need to reduce the sidewall height (actually called the Aspect Ratio) as the tire gets wider so that the same overall diameter is selected as is installed on the front. Much of trhe reason for this is that the traction control looks at the speed of the rear tires relative to the speed of the front and decides that the rear tires have broken loose -- lost traction -- and are spinning on the ground. The traction control will either apply what it does at the wrong time or fail to apply it at the right time depending on variances in the input from the wheel speed sensors that arise from changes in the tire diameter. If the rear tire is smaller than the front, then the traction control will always be on the verge of detecting wheel spin, and you experience drag and performance lags caused by the brakes applying themselves and the throttle shutting down. If the rear tires are larger, then the traction control will take longer to apply itself because the speed of the larger tire has to catch up to and surpass the speed of the front tires before traction control comes into play.

The wheel speed sensors are devices that look at a chopper disc built into the wheel hub. The sensors create a square wave with a frequency that is determined by the speed of the tire on the ground. The design is such that the front and rear should make the same size wave -- the waves should be the same frequency -- at all times. As a practical matter, a car that is turning a corner makes a different frequency square wave at each tire because the arc of the travel is different but the time to travel is the same -- all four tires take a different path to get to the same place so the speed of each is different. The variance caused by turning is built into the algorithm used to determine when a tire is freely spinning. Get it?

So, your traction control light is far more likely to be caused by the work that was done in the area of the sensors than by the tires themselves. You can buy two tires at any time, but they should be the same brand and size when mounted to the same axle -- and it is sheer insanity to put two new tires on one side of the car and have two old tires on the other side, your should not do this, and a reputable tire store will not do it.
I have 2 Pirelli P6 tires 245-40x18 on the rear axel. I just got 2 Falken Ziex-512 for the front axel measuring 225-40x18. So you think this doesn't have much to do with the lights? More of a chance they messed something up?
Before the front was Nexen N3000 of the same dimensions on the front. And as I've said I ran it for six months with no lights at all.
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